Interpretive Summary: Seepage causes the loss of >100 million acre-feet of water from unlined irrigation canals worldwide. Development of inexpensive canal sealing technologies and more efficient, water conserving, surface irrigation systems is hindered by our limited understanding of basic seepage processes in channeled flows. The effect of water borne sediment and polymer amendments on seepage from unlined channels is little understood. However, this information is important if we wish to learn how such processes may be managed to control seepage water losses. This study shows that seepage from these unlined channels can be significantly altered relative to untreated controls by manipulating the sediment particle size and concentration, and PAM concentration of the water inflows. These controlling factors interact in complex ways to control seepage loss. The information is important to on-going research because it identifies factors that critically effect seepage processes and suggests directions for future experimentation aimed at developing water conservation tools.

Technical Abstract:
Seepage from water streams into unlined channels determines the proportion of water distributed to adjacent soil for plant use or soil or groundwater recharge, or conveyed to downstream reaches. We conducted a laboratory study to determine how inflow amendment and related factors, sediment type (none, clay, silt), sediment concentration (0, 0.5, and 2 g/L), and water soluble, anionic polyacrylamide (PAM) concentration (0, 0.4, 2 mg/L) influenced seepage loss from unlined channels in silt loam soil. In a miniflume a preformed channel with 7% slope was supplied with 40 mL/min simulated irrigation water inflows containing the different treatment combinations. Runoff and seepage rates and runoff sediment were monitored for 24-h. When averaged over all treatments, 23-h cumulative seepage loss for inflow containing Silt (11.8 L) was 4.2x that of Clay (2.8 L) and 2.3x that of inflow without sediment (6.4 L) treatments. Increasing inflow clay concentrations produced a 99% linear decrease in seepage volume for the No-PAM treatment (12.4, 6.7, 0.2 L respectively). Increasing inflow PAM concentrations increased seepage volumes for 2-g/L Silt and 2-g/L Clay treatments, but decreased seepage for the 0.5-g/L Silt treatment. Seepage losses from these unlined channels can be significantly altered relative to untreated controls by manipulating the sediment particle size and concentration, and PAM concentration of irrigation water inflows. Their effects on induced seepage changes are complex, strongly controlled by factor interactions, and appear to involve a number of mechanisms, several of which are little understood.